An undercut drill apparatus has been known as this kind of diameter expansion drill bit which is used to be inserted into a straight-shaped prepared hole perforated in a framework such as concrete and expands an innermost of the prepared hole (see Patent Document 1).
The undercut drill apparatus has a hollow cylindrically-shaped cylindrical body inserted into the prepared hole, an abutting member which abuts on an opening edge portion of the prepared hole and rotatably supports the cylindrical body via a bearing, a shaft which slidably engages with the cylindrical body coaxially and rotates with the cylindrical body integrally, a frustum-shaped corn section which is provided at a tip side of the cylindrical body and has four guide grooves on an outer circumferential surface, four arms which are attached on a tip portion of the shaft and engages with each guide groove, and two blades and two guide sections which are provided on an outer surface of a tip portion of the four arms alternately.
The blades and the guide sections locate inside the cylindrical body with a state that the shaft is pulled up when the cylindrical body inserted in the prepared hole and the shaft are rotated integrally and the shaft is moved downwardly, the four arms move downwardly and open outwardly along the guide grooves of the corn section. Thus, the blades grind an inner circumferential surface of the prepared hole to form a diameter expansion portion at a bottom (the innermost) of the prepared hole.
[Patent Document 1] JP-A-2005-280243